The present disclosure is concerned with patient support apparatus and particularly with a patient detection system for detecting the presence of a patient on a patient-carrying surface of the patient support apparatus. Embodiments disclosed herein relate to apparatus for determining the force or pressure applied to the support deck by a patient on the patient support apparatus and means to thereby determine a location for the center of gravity of a patient on the patient support apparatus.
It is sometimes desirable to monitor the whereabouts of a patient, particularly when a doctor or other care giver has ordered that the patient remain on a patient-support device such as a hospital bed, a stretcher, or other device for carrying the patient. Several devices have been developed for detecting a patient exit from a patient-support device. For example, U.S. Pat. No. 5,276,432 to Travis discloses a bed exit detection mechanism for a hospital bed that relies on signals from load cells under the mattress supporting portion on the upper frame of the bed. The load cells support the upper bed frame and also any loads which are in turn supported by the frame, including a patient. This device uses the weight measured by each load cell to determine whether the center of gravity of the patient is over a predetermined region of the patient-carrying surface of the hospital bed. In addition, U.S. Pat. Nos. 4,934,468 and 4,953,244, disclose a hospital bed having a support frame and a weigh frame mounted on the support frame by load cells.
Arrangements such as that of U.S. Pat. No. 5,276,432 comprising a weigh frame or scales under the upper bed frame require expensive components and are therefore expensive. They are also difficult to retrofit to existing beds.
It is also known to monitor the position of a person by attaching a portion of a transmitter/receiver system to the person being monitored so that when the person and thus the portion of the system attached to the person leaves a designated area, the remaining portion of the system detects the departure of the portion and thus the departure of the person and provides an indication of the person's departure. For example, U.S. Pat. No. 5,519,380 to Edwards discloses a bracelet module that is worn by the monitored person so that when the person and the bracelet module move outside of the monitored volume, an indication of departure is provided.
It is also known to monitor the position of a patient on a bed using bladders or other fluid-carrying devices positioned to lie between the patient and the top of the mattress or other patient support surface of the bed. Such bladders or fluid-carrying devices are in fluid communication with a pressure sensor so that the pressure sensor registers a bladder pressure in response to the patient's weight, the bladder pressure indicating the presence or absence of the patient on the bladder. For example, U.S. Pat. Nos. 5,140,309 and 5,184,122, both to Gusakov, each disclose an apparatus including resilient means in the form of a tube, cell or other form of fluid passage adapted to allow fluid such as air to flow from an inlet through the outlet when a passage is not collapsed by the weight of a patient. Indicating means indicate changes in pressure in the fluid supplied by the fluid supplying means, such as when the weight of a patient collapses the passage through the resilient means. By providing resilient means having a plurality of tubes, cells or other fluid passages and separate indicating means for each resilient means, the position of the patient relative to each resilient means can be monitored.
There are also several known systems that include sensors positioned to lie immediately beneath the patient (between the patient and the mattress or other patient support surface) and that provide electrical signals in response to the weight detected by the sensor so that an output signal indicating a significant change of weight acting against the sensor indicates movement of the patient to a position away from the sensor or to a position on top of the sensor. For example, U.S. Pat. No. 5,353,012 to Barham et al.; Swartout disclose a system which includes a sensor that provides an output signal in response to a change in the weight acting against the sensor.
Finally, it is also known to provide capacitive motion sensors for monitoring the movements of a person and even for measuring respiration, heartbeat, and body position of the person. For example, U.S. Pat. No. 4,320,766 to Alihanka et al. and U.S. Pat. No. 5,448,996 to Bellin et al. each disclose patient monitors including capacitive sensors. The device disclosed by the Alihanka patent can indicate that movement has occurred but cannot indicate what movement occurred or the position of the body when no movement is occurring, and the Bellin patent likewise can indicate movement but not position of the patient or that a patient has exited the bed.
Improvements in bed mattress designs have significantly reduced peak interface pressure between the patient and the mattress by maximizing the area over which the weight of the patient is distributed rather than concentrating the interface pressure at points along the patient, for example, the patient's head, shoulder blades and heels. In addition, recent patient population data indicates that some patients in general weigh less than in the past. Patient detection systems that rely on the weight of the patient to allow the sensor to detect the patient are rendered less effective as mattresses become more efficient at distributing the weight of the patient across the surface of the mattress and as the weight of the patient decreases. In addition, while it is desirable to minimize the interface pressure of high interface pressure points between the patient and the mattress by distributing the weight of the patient across the mattress, for example, by using mattresses including air bladder supports, interposing a sensor between the top surface of the mattress and the patient significantly reduces the effectiveness of the mattress at distributing the weight of the patient. In addition, for applications in which it is desirable to keep the patient dry and maintain the temperature of the patient at a desired temperature through the use of a “low air loss” mattress that allows a very small amount of air to escape the mattress and blow on the patient, interposing a sensor between the patient and the mattress reduces the effectiveness of the low air loss feature.
What is needed is a sensor that can sense the position of a patient relative to a patient-carrying surface of a patient-support device that can be positioned to lie away from the patient allowing the sensor to be placed away from the patient-carrying surface. In addition, the sensor should not require the attachment of a portion of the system to the patient.
U.S. Pat. No. 6,067,019 to Scott discloses a bed exit detection apparatus including a number of bed position sensors arranged underneath the mattress between the mattress and the mattress support frame or surface. The sensors each comprise separated conducting plates which together form a capacitor having a capacitor gap whose dielectric consent is affected by the absence or presence of a portion of a human body arranged above, but separated by the mattress from the respective sensor.
The complicated dielectric/capacitive sensor arrangement of Scott was considered necessary in order to overcome the perceived difficulties associated with sensing for the absence or presence of a body remote from the sensors (i.e. separated therefrom by the mattress) and in which the presence of the mattress would distribute the weight of the patient and thereby create difficulties.